AutoCAD drawings are forward compatible only.
The drawing you have must be saved as a 2007 file format and hence is unreadable in AutoCAD 2006.
You need to get the drawing to open in AutoCAD 2008 or later and save the file as a 2004 version file.
Only then will you be able to work with the drawing using AutoCAD LT 2006.
Vivek Ramesh
Autodesk Product Support
Vivek Ramesh
Product Support Specialist
Global Services
Autodesk, Inc.